Steam-cracking of modified naphtha

ABSTRACT

Disclosed is a method for steam-cracking naphtha, according to which a charge of hydrocarbons containing a portion of paraffinic naphtha, which is modified by adding a combination of a first component containing a portion of gasoline and a second component containing a portion of at least one hydrocarbonated refinery gas, and a paraffin-rich change containing at least one paraffin selected among propane, butane, or a mixture thereof are fed through a steam cracker in the presence of vapor. Also disclosed is a hydrocarbon composition suitable for steam cracking, containing a portion of a paraffinic naphtha, which is modified by adding a combination of a first component containing a portion of gasoline and a second component containing a portion of at least one hydrocarbonated refinery gas and a paraffin-rich charge containing at least one paraffin selected among propane, butane, or a mixture thereof.

The present invention concerns a method for steam-cracking naphtha, acomposition of hydrocarbons suitable for steam-cracking, a method forcontrolling a steam cracker, instrumentation for controlling a steamcracker, and a method for processing a sulfurous gasoline feedstock.

The petrochemical industry requires monomers (building blocks) composedfor example of olefins, diolefins and aromatics. In Europe, olefins areprimarily obtained by steam-cracking feedstocks obtained fromrefineries. The available feedstocks are primarily naphtha includingparaffins, isoparaffins and aromatics. A naphtha feedstock usable insteam-cracking is known in the industry as including a petroleumfraction the lightest components of which include five carbon atoms andwhich have a final boiling point of around 200° C., the naphthaincluding high carbon-index components having a boiling point of atleast 200° C. The steam-cracking of naphtha yields light olefins such asethylene and propylene, and diolefins such as butadiene, as well asgasolines containing aromatics.

When a typical naphtha is subjected to steam-cracking, the crackedproduct typically has the following composition (in % by weight) when itleaves the furnace:

% by weight (approx.) Hydrogen 1 Methane 16 Acetylene 0.2 Ethylene 22Ethane 5 Methylacetylene, 0.3 Propadiene Propylene 14 Propane 0.5Butadiene 4 C4 5 C5 4 Benzene 9 Toluene 5 Non-aromatic gasoline 2Aromatic gasoline 6 Fuel oil 6 Total 100

The most interesting fractions in the cracked product are the lightolefins, that is, the ethylene and propylene. Their yield is directlyrelated to the presence of paraffins in the feedstock. When paraffins instraight chains are present, the formation of ethylene is favored. Whenisoparaffins are present, the formation of propylene is favored. Therelative yield in propylene is expressed as the weight ratio of thepropylene to the ethylene, and is typically between 0.5 and 0.75.

Recently, because of the increased need for olefins, the supply of theparaffinic naphtha feedstock in a petrochemical plant fed from arefinery has tended to be somewhat limited.

DE-A-3708332 describes a method of thermal cracking ethylene in a steamcracker, where the ethylene is mixed with the naphtha in order toprepare a feedstock composed essentially of naphtha and 10 to 80% byweight ethylene, optionally containing, in addition to the naphtha,fractions from gas-oil (boiling temperature up to 350° C.) and/orrecycled byproducts from a petrochemical plant that can have up to 50%naphtha. This method has the disadvantage that it requires relativelylarge quantities of ethylene (at least 10%) in the raw materials, andthe yields of ethylene (compared to the ethylene introduced into thefeedstock) and propylene are not particularly high.

U.S. Pat. No. 3,786,110 describes a method of producing unsaturatedhydrocarbons obtained by pyrolysis, where the undesirable fractions arereduced by adding to the pyrolysis products a polymerization inhibitorcontaining asphaltic hydrocarbons.

A method of steam-cracking naphtha capable of furnishing a commerciallyacceptable yield of olefins, in particular light olefins such asethylene and propylene, while reducing the quantity of paraffinicnaphtha feedstock material required, is therefore necessary in thetechnology.

Refineries produce a broad range of products. Depending on the technicaldemands of the local markets and other commercial considerations, someof these products can be of little commercial value and are thereforeconsidered to be “surplus.” Currently products like gasolines andcertain gaseous hydrocarbons are considered to be obtained in quantitiesthat are too large. Although products of this type can be used incertain petrochemical processes, they are not currently used insteam-cracking operations because, for liquid products, they do not havethe required quantity of paraffins.

Ethane and propane are used as feedstocks for steam-cracking, especiallyin the United States where natural gas, from which they are extracted,is abundant. These paraffins generate a large quantity of ethylene (morethan 50%) when they are steam cracked, which results in processing thesefeed-stocks in units that are specifically sized for this type offeedstock. Some refinery hydrocarbon gases such as FCC gases containsubstantial quantities of paraffins (ethane and propane) and olefins(ethylene, propylene). However, when they are steam cracked as such,they have a tendency to generate cracked gaseous effluents having acomposition that is different from that of the effluents fromsteam-cracking normal naphtha. This poses a problem, because itgenerates an imbalance in the downstream section (especially thedistillation columns) of a steam cracker cracking naphtha.

Butane and propane are also used, either alone or in mixture with thenaphtha, as feedstock for steam crackers. When they are usedexclusively, however, the problem of imbalance in the downstream sectionof a naphtha steam cracker also becomes apparent. Depending on theavailability of the refinery or the market, there can be an excess ofthese liquefied gases and it is therefore worthwhile to use them asfeedstock for a steam cracker.

DE-A-3708332, already cited, does not deal with the technical problemthat consists of producing an effluent that has a composition thatcorresponds with the one produced by steam-cracking naphtha. In theexamples of DE-A-3708332, when the ethylene is added (alone) to thenaphtha, the composition of the effluent, particularly with regard toethylene and propylene, is substantially altered as compared to thecracking of naphtha alone under the same conditions, which can result insignificantly reducing the capacity of the steam-cracking unit.

A petrochemical process that increases the economic value of the“surplus” refinery products, such as gasolines and gaseous hydrocarbons,is also necessary for the technology.

The invention seeks at least partially to meet these needs.

To that end, the invention proposes a method for steam-cracking naphtha,said method comprising the passage in a steam cracker, in the presenceof steam, of a feedstock of hydrocarbons comprising a paraffinic naphthamodified by the addition of the combination of a first componentincluding a gasoline and of a second component including at least one ofa hydrocarbon refinery gas and a feedstock rich in paraffins includingat least one paraffin selected from propane and butane or a mixturethereof.

The invention also proposes a composition of hydrocarbons appropriatefor steam-cracking, comprising a paraffinic naphtha modified by theaddition of the combination of a first component including a gasoline,and of a second component including at least one of a hydrocarbonrefinery gas and a feed-stock rich in paraffins including at least oneparaffin selected from propane and butane or a mixture thereof.

The invention also proposes a method for controlling a steam cracker,said method comprising the supply to a steam cracker of steam and of afeed-stock of hydrocarbons comprising a paraffinic naphtha modified bythe addition of the combination of a first component including agasoline, and of a second component including at least one of ahydrocarbon refinery gas and a feed-stock rich in paraffins including atleast one paraffin selected from propane and butane or a mixturethereof, and the continuous control of the supply of paraffinic naphtha,of the second component and of the gasoline in the feedstock, in orderto give the effluent a desired target composition.

Moreover, the invention proposes an instrumentation for controlling asteam cracker, said instrumentation comprising means of furnishing asteam cracker with a hydrocarbon feedstock comprising a paraffinicnaphtha modified by the addition of the combination a gasoline and of atleast one of a hydrocarbon refinery gas and/or butane or propane or amixture thereof, and means for continuously controlling the proportionsof the paraffinic naphtha, of the refinery gas and/or of butane orpropane or the mixture thereof, and of the gasoline in the feedstock, inorder to give the effluent a desired target composition.

The invention also proposes a method for processing a feedstock ofsulfurous gasoline, which method includes the following phases:combining a sulfurous gasoline feedstock with a naphtha feedstock toobtain a composite feed-stock; causing the composite feedstock to passinto a steam cracker, in the presence of steam, to produce an effluent,the effluent containing at least light olefins, the light olefins havingat least one olefin between C2 and C4, and C5+ hydrocarbons; andseparating from the effluent a first fraction that is practically freeof sulfur and includes the light olefins, and a second fraction thatcontains sulfur and includes the C5+ hydrocarbons.

The invention is based on the surprising discovery by the Applicant thatby selecting certain quantities and qualities of these gasolines andgaseous hydrocarbons and by using them as feedstocks in combination withnaphtha, it is possible to steam crack the composite feedstock in orderto produce a composition for the cracked product (called “pallet ofproducts” in the industry) which strongly resembles a pallet of productsresulting from the steam-cracking, under similar conditions, of afeedstock of just paraffinic naphtha. The composition of the effluentproduced according to the invention falls within a range of ±90%, andpreferably ±0% by weight, for each component, compared to that of theeffluent, when said effluent is non-modified paraffinic naphtha.

In reality, therefore, according to the invention part of the paraffinicnaphtha feedstock is replaced by a combination of a gasoline feedstockand a hydrocarbon refinery gas feedstock and/or a butane or propanefeedstock or mixture of the two.

This offers the combined advantages of (a) reducing the quantity ofparaffinic naphtha feeds needed for the steam-cracking process, and (b)using the gaseous hydrocarbon products and “surplus” gasoline productsin the steam-cracking process to produce products that are economicallybeneficial and useful, to wit, light olefins, while making only minormodifications to the steam-cracking unit since the overall balance ofmaterials is only slightly modified.

Forms of embodiment of the invention will now be described, solely byway of example, with reference to the appended drawing in which:

FIG. 1 diagrammatically shows a unit for the steam-cracking offeedstocks containing naphtha according to one form of embodiment of theinvention.

According to the invention, a method for the steam-cracking of naphthauses a feedstock composed of hydrocarbons, comprising a paraffinicnaphtha modified by a gasoline in combination with a hydrocarbonrefinery gas and/or butane or propane or a mixture thereof.

The paraffinic naphtha to be used in the method of the inventionincludes 10 to 60% by weight of n-paraffins, 10 to 60% by weight ofisoparaffins, 0 to 35% by weight of naphthenes, 0 to 1% by weight ofolefins and 0 to 20% by weight of aromatics. A typical paraffinicnaphtha to be used in the method of the invention includes about 31% byweight of n-paraffins, 35% by weight of isoparaffins (giving a totalparaffinic content of 66% by weight), 26% by weight of naphthenes, 0% byweight of olefins (typically 0.05% by weight of olefins) and 8% byweight of aromatics.

According to the invention, this starting feedstock of paraffinicnaphtha is modified by adding to it a gasoline and a hydrocarbonrefinery gas and/or butane or propane or a mixture thereof.

The gasoline is preferably a fraction from an FCC (fluidized-bedcatalytic cracking) unit of an oil refinery (hereinafter called FCCgasoline), which advantageously has not been given a hydrogenationtreatment (called “hydro-refining” in the industry), which increases theparaffins content of the gasoline by hydrogenating the unsaturatedfunctions (like those present in the olefins and diolefins) of thegasoline. The advantage of using a non-hydro-refined FCC gasoline isthat by avoiding a hydrogenation process, production costs are reducedby eliminating or reducing the use of hydrogen and by avoiding the needfor additional hydro-refining capacity.

The FCC gasoline is a fraction or a mixture of fractions from the FCCunit typically having a distillation range of between 30 and 160° C.,preferably a fraction or mixture of fractions that reach boiling in arange of between 30 and 65° C., 65 to 105° C. and 105 to 145° C. Thechoice of the particular FCC gasoline or mixture thereof to be used canbe determined based on the requirements at any time for the variousfractions produced by the refinery. For example, some gasoline fractionshave an octane deficit and could be better upgraded in a steam crackerrather than having to increase the octane index in the refinery. Inaddition, the FCC gasoline to be used can have a sulfur content thatwould be too high for gasolines to be used in automobiles and wouldrequire hydrogen desulfurization treatment, which is costly because itconsumes hydrogen and requires the corresponding capacity on adesulfurization unit.

It is preferable to use a non-hydrorefined FCC gasoline because, in therefinery, where there is a need for hydrorefined gasoline for otheruses, this can cause bottlenecks in the processing by the hydro-refiningunit. By reducing the quantity of non-hydro-refined gasoline present inthe refinery, that is, by consuming the non-hydro-refined gasoline inthe steam-cracking process of the invention, the hydro-refiningequipment and units can be unblocked, thus improving the management offlows in the refinery while reducing the needs for hydrogen.

Typically the non-hydro-refined FCC gasoline includes from 0 to 30% byweight of n-paraffins, from 10 to 60% by weight of isoparaffins, from 0to 80% by weight of naphthenes, from 5 to 80% by weight of olefins andfrom 0 to 60% by weight of aromatics. More typically, thenon-hydrorefined FCC gasoline includes approximately 3.2% by weight ofn-paraffins, 19.2% by weight of isoparaffins (giving a total paraffinscontent of 22.4% by weight), 18% by weight of naphthenes, 30% by weightof olefins and 29.7% by weight of aromatics.

However, if a hydro-refined FCC gasoline were used, a substantialquantity of hydrogen would be needed to hydrogenate it and thehydrorefined composition would resemble typical naphtha used forsteam-cracking.

With regard to the hydrocarbon refinery gas that is added, incombination with the FCC gasoline and/or the butane or propane or amixture of the two, to the paraffinic naphtha to produce a compositefeedstock for the steam-cracking, this hydrocarbon gas is rich in C₂ andC₃ hydrocarbons, particularly in paraffins (ethane and propane) and inolefins (ethylene and propylene). Preferably the refinery gas has thefollowing composition ranges: 0 to 5% by weight of hydrogen, 0 to 40% byweight of methane, 0 to 50% by weight of ethylene, 0 to 80% by weight ofethane, 0 to 50% by weight of propylene, 0 to 80% by weight of propaneand 0 to 30% by weight of butane. A typical composition of refinery gasof this type is approximately 1% by weight of hydrogen, 2% by weight ofnitrogen, 0.5% by weight of carbon monoxide, 0% by weight of carbondioxide, 10% by weight of methane, 15% by weight of ethylene, 32% byweight of ethane; 13% by weight of propylene, 14% by weight of propane,2% by weight of isobutane, 4% by weight of n-butane, 3% by weight ofbutene, 2% by weight of n-pentane, and 1.5% by weight of n-hexane.

With regard to the butane and/or propane or the mixture thereof that isadded to the paraffinic naphtha, in combination with the FCC gasolineand optionally the refinery gas, in order to produce a compositefeedstock for steam-cracking, this butane and/or propane or the mixturethereof can contain olefinic compounds such as butenes and/or propylene,or saturated compounds such as butanes (normal and/or iso) and/orpropane. Preferably the butane and/or propane or the mixture thereofcontain more than 50% by weight of saturated compounds in order tomaximize the production of light olefins such as ethylene and propylene.The butane and propane are preferably n-butane and n-propane.

According to the method of the invention, the parts of naphtha, refinerygas, butane or propane or a mixture thereof, and gasoline are combinedto form a composite feedstock that is then processed by steam-cracking.Preferably, the composite feedstock includes from 5 to 95% by weight ofnaphtha, 5 to 95% by weight of a mixture of refinery gas, butane orpropane or a mixture of thereof, and gasoline. Typically, the mixture ofrefinery gas, butane or propane or a mixture thereof, and gasoline thatis added to the naphtha includes up to 60% by weight of refinery gasand/or butane or propane or a mixture thereof, and at least 40% byweight of gasoline, more typically up to 50% by weight of refinery gasand/or butane or propane or a mixture thereof, and up to 50% by weightof gasoline. In a more preferred manner, the naphtha composite includes80% by weight of naphtha, 7% by weight of refinery gas and/or butane orpropane or a mixture thereof, and 13% by weight of non-hydrorefined FCCgasoline.

The composite feedstock of naphtha, gasoline, refinery gas and/or butaneor propane or a mixture thereof, is typically processed bysteam-cracking under conditions similar to those known in the industry,to wit, at a temperature of between 780 and 880° C., preferably between800 and 850° C. The quantity of steam can also fall within a range knownin the industry, typically between 25 and 60% by weight on the basis ofthe weight of the hydrocarbon feedstock.

With reference to FIG. 1 of the appended drawing, the hot section of asteam cracking unit to be used in the method of the invention isrepresented diagrammatically. The steam cracking unit, generallyindicated by 2, includes a heating unit composed of furnaces 4, whichare provided with coils 6 having a first intake 8 for the feedstock ofhydrocarbons to be cracked and a second intake 10 for the steam. Anoutlet conduit 12 from the heating unit is connected to a firstfractionating column 14. The first fractionating column 14 includes agasoline reflux 15 and outlets for the various fractionated products,including a top outlet 16 for the light hydrocarbons and a bottom outlet18 for the heavy hydrocarbons, which can be returned through 19 aftercooling in the conduit 12 in order to control the temperature or drawnoff at 17 in the form of heavy products called pyrolysis oil.

In this simplified description and in the presentation of the followingexamples, only the feeds from outside the steam cracker, commonly calledfresh feeds, are considered, and not the possible recycling of productscoming from the steam cracker itself, such as ethane, which is oftenre-cracked until it disappears.

If desired, the entire composite feedstock of naphtha, refinery gasand/or butane or propane or a mixture of the two, and gasoline can befed through the common intake 8 of hydrocarbons, or alternatively, thefour components naphtha, FCC gasoline, refinery gas and/or butane orpropane or a mixture thereof can be cracked separately in specifictubular coils. In one particular form of embodiment, the naphtha and theFCC gasoline, for the one part, the butane and/or propane or a mixtureof the two, and the refinery gas, for the other part, are crackedseparately. The reason for this is that the naphtha and the FCC gasolineare typically cracked a temperatures that are close to each other,typically within the range of 750 to 850° C., while the butane, propaneand refinery gases that contain the ethane and propane should be crackedat higher temperatures, typically within the range of 800 to 900° C. Thetwo effluents can be combined at the outlet of the heating unit beforethe first fractionating column.

The method of the invention can function continuously and offers theadvantage of eliminating the excess gasoline from the refinery, and alsoreducing the need at the refinery for a desulphurization process. Thegasoline contains sulfur, and after the steam-cracking process, in whichthe gasoline furnishes part of the feedstock, the most-interesting lightolefins in the effluent are free of sulfur, while the sulfur remainsconcentrated in the C5+ part of the effluent flow. Consequently the useof gasoline as part of a feedstock to be steam cracked to produce thelightest olefins leads to a partial desulfurization of the gasolineportion of the feedstock, because the sulfur is concentrated in thefraction with the highest carbon number and of least commercial interestof the effluent, to wit, the C5+ flow.

In a related way, according to another aspect, the invention proposes amethod for processing a sulfurous gasoline feedstock, which methodcomprises the following phases: combining a sulfurous gasoline feedstockwith a naphtha feedstock to provide a composite feedstock; causing thecomposite feedstock to pass through a steam cracker, in the presence ofsteam, to produce an effluent, the effluent containing at least lightolefins, the light olefins having at least one of the C2 to C4 olefins,and C5+ hydrocarbons; and separating from the effluent a first fractionthat is practically free of sulfur and includes the light olefins, and asecond fraction that contains the sulfur and includes the C5+hydro-carbons. In this way, the sulfur is redistributed in the highercarbon number fraction, producing a lower carbon number olefinicfraction free of sulfur, which is an effective way to partiallydesulfurize the gasoline feedstock.

Moreover, the method offers the advantage that the steam-crackingprocess at least partially dehydrogenates the ethane present in therefinery gas, the dehydrogenation being accomplished at a temperaturehigh enough to effectively produce ethylene.

The invention also offers the advantage that by adding to the feedstocknaphtha that contains no olefins or only a small quantity thereof,non-hydro-refined gasoline that contains a relatively high quantity ofolefins, typically from 5 to 80% by weight of olefins; the compositefeedstock for the steam-cracking has a higher overall content of olefinscompared to the naphtha alone, and this results in a lower use of energyto produce light olefins (that is, ethylene and propylene) from thisfeedstock, compared to steam-cracking paraffins or paraffinic feedstocksinto light olefins of this type.

According to another aspect of the invention, a software program, usinglinear or non-linear programming, is used to continuously control thesteam-cracking conditions, in particular to control the parts of theparaffinic naphtha, the refinery gas, the butane and/or propane or themixture of the two, and the FCC gasoline in the feedstock, so that theeffluent has the desired target composition. For example, the targetcomposition can have appreciably the same effluent composition for theimportant components, that is, ±20% by weight, preferably ±10% by weightcompared to that of the unmodified feedstock. The software can alsocontrol the flow of the refinery gas and/or control the quantities ofFCC gasoline and/or butane or propane or mixture of the two, taken fromthe refinery for shipment of excess quantities, for example, to storagetanks.

The invention will now be described in more detail with reference to thefollowing two examples.

Example 1

In this example, a composite feedstock having 80% by weight naphtha and20% mixture of refinery gas and non-hydrorefined FCC gasoline, at aratio by weight one third gas and two thirds gasoline, was subjected tosteam-cracking.

The naphtha has the following approximate starting composition:

-   -   31% by weight n-paraffins,    -   35% by weight isoparaffins (giving a total paraffinic content of        66% by weight),    -   26% by weight naphtalenes,    -   0.05 wt olefins,    -   0% by weight diolefins,    -   8% by weight aromatics.

The refinery gas has the following approximate starting composition:

-   -   1% by weight hydrogen,    -   2% by weight nitrogen,    -   0.5% by weight carbon monoxide,    -   0% by weight carbon dioxide,    -   10% by weight methane,    -   15% by weight ethylene,    -   32% by weight ethane,    -   13% by weight propylene,    -   14% by weight propane,    -   2% by weight isobutane,    -   4% by weight n-butane,    -   3% by weight butene,    -   2% by weight n-pentane,    -   1.5% by weight n-hexane.

The non-hydro-refined FCC gasoline has the following approximatestarting composition:

-   -   3% by weight n-paraffins,    -   19% by weight isoparaffins (giving a total paraffinic content of        22% by weight),    -   18% by weight naphthenes,    -   30% by weight olefins,    -   30% by weight aromatics.

After steam-cracking, the overall effluent from the set of furnaces atoutlet 12 without recycling the ethane produced by the steam cracker,has the composition shown in Table 1.

TABLE 1 Composition of the Effluent of Example 1 % by weight (approx.)H₂ 0.9 Methane 16.0 Acetylene 0.2 Ethylene 22.0 Ethane 5.3Methylacetylene-propadiene 0.3 Propane 0.6 Propylene 12.5 Butadiene 3.4C4 4.4 C5 3.8 Benzene 8.9 Toluene 6.3 Non-aromatic gasoline 2.0 Aromaticgasoline 6.9 Fuel oil 6.5

By contrast, when 100% of the same naphtha is subjected tosteam-cracking under the same conditions, the effluent obtained had thecomposition shown in Table 2.

TABLE 2 Non-hydro-refined Naphtha Refinery Gas FCC gasoline Hydrogen 0.82.6 0.6 Methane 15.2 27.4 13.6 Acetylene 0.2 0.2 0.1 Ethylene 21.8 43.512.5 Ethane 5.0 12.5 3.1 MAPD 0.4 0.1 0.3 Propylene 14.2 2.7 7.5 Propane0.6 0.5 0.3 Butadiene 3.7 1.7 2.2 C4 5.1 0.4 2.5 C5 4.3 0.6 2.2 Benzene9.1 3.8 10.0 Toluene 5.4 0.5 14.9 Non-aromatic gasoline 2.4 0.1 1.1Aromatic gasoline 5.8 1.4 16.8 Fuel oil 6.0 2.0 12.3

As can be seen, the effluent resulting from the steam-cracking of thecombination of three feedstocks of paraffinic naphtha, refinery gas andnon-hydro-refined FCC gasoline very strongly resembles the effluentproduced by steam-cracking paraffinic naphtha alone.

Thus, the composition of the effluent from the composite feedstock ofExample 1 is similar (±10% by weight for each component) to that ofnaphtha alone, but part of the naphtha has been replaced by the additionof the refinery gas and the FCC gasoline for the reasons and advantagesindicated above. It can be seen that the high yields of ethylene andpropylene obtained by the method according to the invention are similarto those that can be obtained simply by steam-cracking paraffinicnaphtha.

Table 2 also shows, by contrast, the compositions of the effluentsobtained by steam-cracking of refinery gas alone, and separately, theFCC gasoline alone. As can be seen, the steam-cracking of thenon-hydro-refined FCC gasoline produces a low yield of ethylene andpropylene, and the steam-cracking of refinery gas produces a high yieldof ethylene, but a low yield of propylene. However, when the threefeedstocks of naphtha, refinery gas and non-hydro-refined FCC gasolineare combined, the composition of the effluent very strongly resemblesthat of normal naphtha.

Example 2

In this example, a composite feedstock having 60% by weight naphtha and40% by weight mixture of butane and non-hydro-refined FCC gasoline, in aratio by weight half gas and half gasoline, is subjected to steamcracking.

The naphtha has the same starting composition as in the precedingexample.

The butane in this example is normal pure butane, as can be produced atthe output of a refinery alkylation unit.

The non-hydro-refined FCC gasoline has the same starting composition asin the preceding example.

After steam cracking, the overall effluent from the set of furnaces atoutlet 12 without recycling the ethane produced by the steam cracker hasthe composition shown in Table 3.

TABLE 3 Composition of the Effluent of Example 2 % by weight (approx.)H₂ 0.8 Methane 15.4 Acetylene 0.2 Ethylene 21.9 Ethane 4.8Methylacetylene-propadiene 0.4 Propylene 14.1 Propane 0.5 Butadiene 3.2C4 5.9 C5 3.7 Benzene 7.7 Toluene 6.3 Non-aromatic gasoline 2.1 Aromaticgasoline 6.9 Fuel oil 6.1

By contrast, when 100% of the same naphtha was subjected tosteam-cracking under the same conditions, the effluent obtained had thecomposition shown in Table 2 and recalled in Table 4.

TABLE 4 Non-hydro-refined Naphtha Butane FCC gasoline Hydrogen 0.8 0.90.6 Methane 15.2 18.8 13.6 Acetylene 0.2 0.4 0.1 Ethylene 21.8 32.7 12.5Ethane 5.0 5.9 3.1 MAPD 0.4 0.3 0.3 Propylene 14.2 19.7 7.5 Propane 0.60.4 0.3 Butadiene 3.7 2.8 2.2 C4 5.1 11.2 2.5 C5 4.3 2.2 2.2 Benzene 9.12.2 10.0 Toluene 5.4 0.6 14.9 Non-aromatic gasoline 2.4 0.8 1.1 Aromaticgasoline 5.8 0.5 16.8 Fuel oil 6.0 0.6 12.3

As can be seen, the effluent resulting from the steam-cracking of thecombination of three feedstocks of paraffinic naphtha, butane andnon-hydro-refined FCC gasoline very strongly resembles the effluentproduced by steam-cracking paraffinic naphtha alone.

Thus, the composition of the effluent from the composite feedstock ofExample 2 is similar (±10% by weight for each component) to that ofnaphtha alone, but part of the naphtha has been replaced by the additionof butane and FCC gasoline for the reasons and advantages indicatedabove. It can be seen that the high yields of ethylene and propyleneobtained by the method according to the invention are similar to thosethat can be obtained simply by steam-cracking paraffinic naphtha.

Table 4 also shows, by contrast, the compositions of the effluentsobtained by steam cracking of butane alone, and separately, the FCCgasoline alone. It can be seen that the steam-cracking of thenon-hydro-refined FCC gasoline produces a low yield of ethylene andpropylene, and that the steam-cracking of butane produces a high yieldof ethylene, propylene and C4, and low yields of heavy products.However, when the three feedstocks of naphtha, butane andnon-hydro-refined FCC gasoline are combined, the composition of theeffluent very strongly resembles that of normal naphtha.

1. A method for steam-cracking naphtha comprising passing a feedstock ofhydrocarbons through a steam cracker, in the presence of steam; whereinsaid feedstock of hydrocarbons comprises about 60% to about 80% byweight of a paraffinic naphtha, and about 40% to about 20% by weight ofa mixture of a first component, that is, a gasoline produced by afluidized bed catalytic cracking (FCC) unit and a second componentincluding at least one hydrocarbon refinery gas and at least a feedstockrich in paraffins, the paraffinic naphtha having from 10 to 60% byweight of n-paraffins, from 10 to 60% by weight of isoparaffins, from 0to 35% by weight of naphthenes, from 0 to 1% by weight of olefins, andfrom 0 to 20% by weight of aromatics, the FCC gasoline being anunhydrogenated gasoline having from 0 to 30% by weight of n-paraffins,from 10 to 60% by weight of isoparaffins, from 0 to 80% by weight ofnaphthenes, from 5 to 80% by weight of olefins and from 0 to 60% byweight of aromatics, the refinery gas having from 0 to 5% by weight ofhydrogen, from 0 to 40% by weight of methane, from 0 to 50% by weight ofethylene, from 0 to 80% by weight of ethane, from 0 to 50% by weight ofpropylene, from 0 to 80% by weight of propane and from 0 to 30% byweight of butanes, and the feedstock rich in paraffins containing atleast 50% by weight of saturated hydrocarbons, said feedstock containingat least propane or butane.
 2. The method according to claim 1, in whichthe mixture of the first and second components that is added to thenaphtha includes up to 60% by weight of the second component and atleast 40% by weight of gasoline.
 3. The method according to claim 2, inwhich the mixture of the first and second components includes up to 50%by weight of the second component and at least 50% by weight ofgasoline.
 4. The method according to claim 2 or 3, in which the mixtureof the first and second components includes about one third inpercentage by weight of the second component and about two thirds inpercentage by weight of gasoline.
 5. The method according to any one ofclaims 1-3, in which the feedstock includes about 80% by weight naphtha,about 7% by weight of the second component and about 13% by weight ofgasoline.
 6. The method according to any one of claims 1-3, in which theFCC gasoline is a fraction or a mixture of fractions from an FCC unithaving a distillation range of between 30 and 160° C.
 7. The methodaccording to any of claims 1 to 3, in which the second componentconsists of at least propane and butane.
 8. The method according to anyof claims 1 to 3, in which the composite feedstock of naphtha, gasolineand of the second component is subjected to steam-cracking underconditions including a temperature of between 780 and 880° C.
 9. Themethod according to any of claims 1 to 3, in which the quantity of steamis from 25 to 60% by weight based on the weight of the feedstock ofhydrocarbons.
 10. A composition of hydrocarbons suitable for asteam-cracking method, said composition comprising about 60% to about80% by weight of a paraffinic naphtha and about 40% to about 20% byweight of a mixture of a first component, that is, a gasoline producedby a fluidized bed catalytic cracking (FCC) unit and a second componentincluding at least one hydrocarbon refinery gas and at least a feedstockrich in paraffins, the paraffinic naphtha having from 10 to 60% byweight of n-paraffins, from 10 to 60% by weight of isoparaffins, from 0to 35% by weight of naphthenes, from 0 to 1% by weight of olefins andfrom 0 to 20% by weight of aromatics, the FCC gasoline being anunhydrogenated gasoline having from 0 to 30% by weight of n-paraffins,from 10 to 60% by weight of isoparaffins, from 0 to 80% by weight ofnaphthenes, from 5 to 80% by weight of olefins and from 0 to 60% byweight of aromatics, the refinery gas having from 0 to 5% by weight ofhydrogen, from 0 to 40% by weight of methane, from 0 to 50% by weight ofethylene, from 0 to 80% by weight of ethane, from 0 to 50% by weight ofpropylene, from 0 to 80% by weight of propane and from 0 to 30% byweight of butanes, and the feedstock rich in paraffins containing atleast 50% by weight of saturated hydrocarbons, said feedstock containingat least propane or butane.
 11. The composition of hydrocarbonsaccording to claim 10, in which the mixture of the first and secondcomponents to the naphtha includes up to 60% of the second component andat least 40% by weight gasoline.
 12. The composition of hydrocarbonsaccording to claim 11, in which the mixture of the first and secondcomponents includes up to 50% of the second component and at least 50%by weight of gasoline.
 13. The composition of hydrocarbons according toclaim 11 or 12, in which the mixture of the first and second componentsthat is added to the naphtha includes about one third in percentage byweight of the second component and about two thirds in percentage byweight of gasoline.
 14. The composition of hydrocarbons according toclaim 13, in which the feedstock includes about 80% by weight ofnaphtha, about 7% by weight of the second component and about 13% byweight of gasoline.
 15. The composition of hydrocarbons according to anyof claims 10 to 12, in which the second component consists of at leastpropane and butane.
 16. A method for controlling a steam cracker, saidmethod comprising: supplying steam and a feedstock of hydrocarbons to asteam cracker; wherein the feedstock of hydrocarbons comprises about 60%to about 80% by weight of a paraffinic naphtha and about 40% to about20% by weight of a mixture of a first component, that is, a gasolineproduced by a fluidized bed catalytic cracking (FCC) unit, and a secondcomponent including at least one hydrocarbon refinery gas and at least afeedstock rich in paraffins, the paraffinic naphtha having from 10 to60% by weight of n-paraffins, from 10 to 60% by weight of isoparaffins,from 0 to 35% by weight of naphtalenes, from 0 to 1% by weight ofolefins and from 0 to 20% by weight of aromatics, the FCC gasoline beingan unhydrogenated gasoline having from 0 to 30% by weight ofn-paraffins, from 10 to 60% by weight of isoparaffins, from 0 to 80% byweight of naphthenes, from 5 to 80% by weight of olefins and from 0 to60% by weight of aromatics, the refinery gas having from 0 to 5% byweight of hydrogen, from 0 to 40% by weight of methane, from 0 to 50% byweight of ethylene, from 0 to 80% by weight of ethane, from 0 to 50% byweight of propylene, from 0 to 80% by weight of propane and from 0 to30% by weight of butanes, and the feedstock rich in paraffins containingat least 50% by weight of saturated hydrocarbons, said feedstockcontaining at least propane or butane; and a continuous control of thesupply of paraffinic naphtha, of the second component and of thegasoline in the feedstock, in order to give an effluent a desired targetcomposition.
 17. The method according to claim 16, in which the targetcomposition is appreciably the same composition of effluent, that is±20% by weight for any given effluent component, as the compositionobtained with unmodified paraffinic naphtha.
 18. The method according toclaim 16 or 17, in which the second component and the gasoline are bothfurnished directly by a refinery and also including control of sendingthe second excess component to the flare and/or the control of thequantity of gasoline in the refinery.
 19. The method according to anyone of claims 16 to 17, in which the supply of the components of thefeedstock to the steam cracker is controlled by software.
 20. The methodaccording to claim 1 or 16 for processing sulfurous gasoline whereinsulfurous gasoline produced by a fluidized bed catalytic cracking unit,and effluent from the steam cracker containing at least light olefins,which have at least one olefin between C2 and C4, and C5+ hydrocarbons,are separated into a first fraction, which is practically free of sulfurand includes the light olefins, and a second fraction that containssulfur and includes the C5+ hydrocarbons.